CPU and ventilator constantly running without any process

I'm using Trisquel 8 / MATE on a lenovo ThinkPad L540. In particular, it's the following:
Version 8.0, Flidas 64-bit; Kernel Linux 4.4.0-176-generic x86_64; MATE 1.12.1
Memory 7,5 GiB; Processor: Intel® Core™ i5-4210M CPU @ 2.60GHz × 4
Free harddisk space: 363,2 GiB.

For a couple of days, I've been experiencing the following: 1 CPU is running permanently; it was the ventilator that drew my attention to it in the first place. MATE system monitor shows no process active other than the monitor itself, all other processes are 'sleeping'. It occurs mainly after closing Icedove (60.9.0) and Abrowser (74.0.1) one after the other. Log out does not help, only reboot.

What is going on? Is it bad? How can I prevent it, in order to save battery power?

Thanks for any help in advance!